A thermal base precursor is a neutral or weakly basic compound which can generate a strong base during thermal processing. Various base precursors are known as, for example, described in U.S. Pat. Nos. 3,220,846; 4,060,420 and 4,731,321. Japanese Patent Application No. 1-150575 describes thermally-releasable bis-amines in the form of their bis(aryl sulfonylacetic acid)salts. Other amine-generating compounds include 2-carboxycarboxamide derivatives disclosed in U.S. Pat. No. 4,088,469, hydroxime carbamates disclosed in U.S. Pat. No. 4,511,650 and aldoxime carbamates disclosed in U.S. Pat. No. 4,499,180. Examples of some thermal base precursors are shown in Table III of U.S. Pat. No. 5,258,274 to Helland et al., including cations and anions, which patent is incorporated by reference.
Further examples of base precursors include salts of carboxylic acids and organic bases as described in U.S. Pat. No. 3,493,374 (triazine compounds and carboxylic acids), British Patent 998,949 (trichloroacetic acid salts), U.S. Pat. No. 4,060,420 (sulfonylacetic acid salts), JP-A-59-168441 (The term “JP-A” as used herein means an “unexamined published Japanese patent application”) (sulfonylacetic acid salts), JP-A-59-180537 (propionic acid salts), JP-A60-237443 (phenylsulfonylacetic acid salts substituted by a sulfonyl group), and JP-A-61-51139 (sulfonylacetic acid salts).
Base precursors consisting of carboxylic acids and organic di or tetra-acidic bases are disclosed in JP-A-63-316760 and JP-A-1-68746 (corresponding to U.S. Pat. No. 4,981,965). In these base precursors, the activity on heat treatment at 140° C. is compatible with the storability. EP0708086 discloses selected base precursors which simultaneously satisfy both the activity on heat treatment at 120° C. or less and the storability.
Base precursors each has an inherent decomposition point. However, in practical applications rapid decomposition of the base precursors (the release of bases) is expected only at heating temperatures much higher than their decomposition points. For example, although ease of the decomposition also is dependent on methods of heating, in order to obtain rapid decomposition at a heating temperature of 120° C., the base precursors must usually have a decomposition point of about 100° C. or less.
Other bisguanidine base precursors that are known are described in EP0708086, hereby incorporated by reference. These base precursors can be employed when it is desirable to rapidly release a base at a low heating temperatures and have good storability at the same time. Such bisguanidine salts are selected from the group consisting of a 4-(phenylsulfonyl)phenylsulfonylacetic acid salt of N,N′bis(1,3-diethylguanyl)ethylenediamine, a 4(phenylsulfonyl)phenylsulfonylacetic acid salt of N,N′-bis(1,3diisopropylguanyl)ethylenediamine, 4(phenylsulfonyl)phenylsulfonylacetic acid salt of N,N′-bis-(imidazoline-2yl)ethylenediamine, and other specified compounds.
Base precursors that are neutral or weakly basic compounds that can form relatively strong bases, in heat developable recording materials, by heat decomposition of the base precursors, are described in U.S. Pat. No. 4,981,965, hereby incorporated by reference. This patent describes base precursors comprising arylsulfonylacetic acid salts of guanidine bases. Such base precursors have a stable crystal structure, which crystal structure is kept until it melts or is dissolved at an elevated temperature. Therefore, the carboxylic acid is rapidly decarboxylated to release a base at the same time that the crystal structure is broken.
When the carboxylic acid has hydrophobic residues, the carboxyl group of the carboxylic acid and the organic base are blocked by the hydrophobic residues in the base precursor of the present invention. Accordingly, the base precursor is prevented by the hydrophobic residue from being dissolved in a binder (which generally is hydrophilic). The crystal structure of the salt is further stabilized by intermolecular interaction between the hydrophobic residues. Therefore, such preferred base precursors for use in the present compositions exhibit much higher stability during storage when the carboxylic acid has the hydrophobic residues. Examples of the carboxylic acid are given in the cited U.S. Pat. No. 4,981,965, in columns 9–10.
U.S. Pat. No. 4,981,965 describes the use of guanidinium salts of arylsulfonylacetic acids as base precursors. The diacidic to tetraacidic base precursors are composed of two to four guanidinium units. In these systems, thermolysis of the salt results in decarboxylation to form a arylsulfonylmethyl anion. This anion abstracts a proton from the guanidinium salt to release the free base. This base can then provide the alkalinity required for a number of image-forming processes.
U.S. Pat. No. 4,060,420 describes the use of ammonium salts of arylsulfonylacetic acids as activator-stabilizers in photothermographic systems. In these systems the ammonium species is always a protonated basic nitrogen, and thus has at least one labile hydrogen atom. U.S. Pat. No. 4,731,321 discloses ammonium salts of arylsulfonylacetic acid as base precursors in heat-developable lightsensitive materials.
Japanese Patent Application No. 1-150575 discloses thermally releasable bisamines in the form of their bis(arylsulfonylacetic acid) salts. Other amine-releasing compounds include 2-carboxycarboxamide derivatives disclosed in U.S. Pat. No. 4,088,496; hydroxylamine carbamates disclosed in U.S. Pat. No. 4,511,650; and aldoxime carbamates disclosed in U.S. Pat. No. 4,499,180.
It is usually desirable for a base precursor to exhibit good stability during storage but to quickly decompose to form a base when it is heated at the temperature of use. A successful base precursor will not have any adverse effects on the adjacent layers.